I use Paint Tool SAI. Great program--if you like GIMP you should like SAI. It's not really editing or effect software like Photoshop, it focuses more on pen/brush tools along with really nice vectoring tools.
http://sai.detstwo.com/sai/
It isn't "free" unless you know where to look, of course. But it's also not retardedly expensive like Adobe programs if you choose to use it legit.